A fast-moving blaze Wednesday night gutted a two-story duplex on Butler’s South Side, prompting an investigation by the state police fire marshal. While investigators would not say if the fire in the 400 block of Fairview Avenue was suspicious, firefighters found at least one suspicious circumstance. The front door of the apartment where the blaze started shortly before 9:50 p.m. was open but no one was home, authorities said. The tenant in the adjoining apartment got out without injury, thanks to an alert passer-by, funeral home director Tom Martin. Damage to the structure, owned by Ben Hetrick of Natrona Heights, Allegheny County, is estimated at $45,000 and another $15,000 to its contents. Trooper David Jungling, a deputy fire marshal, was at the scene later that night to begin his investigation. He could not be reached for comment this morning. Police may launch their own investigation, depending on what Jungling determines started the fire.